I posted a question a couple of months ago that got several answers. At the time, I clicked to "accept" the one that seemed best to me, and when I did I got a +2 bonus to my reputation.
In the time since then, I've been reading things that have helped me to come to a much more comprehensive answer to my question than any of those that were posted, so I returned and added my own answer.
According to the FAQ, this is a good thing to do. The Stack Exchange blog even says that
it is not merely OK to ask and answer your own question, it is explicitly encouraged.
Of course if I'm bothering to return after months to post my own answer, that answer will be my new "accepted" one. Yet when I selected my new answer, I found that it gave -2 to my reputation.
I suppose the philosophy behind this must be that it wouldn't make sense for users to be able to auto-generate their own reputation by just creating and accepting their own answers, but two reputation is such a small amount that I doubt this would present a realistic problem. Instead what happens is it feels like a slap on the hand to lose a small amount of reputation in return for having bothered to provide a more thorough answer to the community.
The interests of the user who posted the answer that I had originally accepted should also be considered. Because she already has several votes in her favor, losing my "accept" vote now makes it possible for her to get the gold Populist badge. She loses fifteen reputation, but that seems a small price to pay for a chance at a gold badge, and I doubt most users would object to being given such an opportunity.
Why do you lose reputation for changing the "accepted" answer to your own? Should this policy be changed?